This invention relates to a variable magnification copying apparatus, and more particularly to such a copying apparatus which is provided with first and second reflector means movable at a velocity ratio of 1:1/2 to scan an image original to be copied.
A copying apparatus in which first and second reflector means movable at a velocity ratio of 1:1/2 optically scan an image original to be copied and the optical image of the original is projected on a movable photosensitive medium in the fashion of so-called slit-exposure is known in the art. The first and second reflector means are moved at the velocity ratio of 1:1/2 for the purpose of maintaining a predetermined length of the optical path between a focusing lens and the original during the scanning period.
On the other hand, if the length of the optical path between the principal plane of the focusing lens and the original is a, the length of the optical path between the principal plane of the focusing lens and the photosensitive medium at the exposure station is b and the focal length of the focusing lens is f, and when there is established a relation that 1/a+1/b=1/f, then an optical image at magnification m of the original is formed on the photosensitive medium. Here, m=b/a. By using m and f, a and b may be expressed as: a=(m+1)f/m and b=(m+1)f. Also, a+b=(m+1).sup.2 f/m.
As can be seen from the foregoing, the lengths of optical paths a, b and (a+b) must be varied when copy magnification is changed. For example, when copy magnification is changed from m to m.sub.c, the lengths of optical paths a, b and (a+b) must be changed to a+.DELTA.a, b+.DELTA.b, and (a+b)+.DELTA.(a+b), respectively. Here, .DELTA.a=(m-m.sub.c)f/mm.sub.c, .DELTA.b=(m-m)f and .DELTA.(a+b)=(m.sub.c -m)(mm.sub.c -1)f/mm.sub.c.
When changing copy magnification in a copying apparatus provided with a scanning optical system comprising first and second reflector means movable at a velocity ratio of 1:1/2, as already noted, it has heretofore been the usual practice to change the location of the focusing lens and to adjust the scanning optical system, thereby changing the aforementioned lengths of optical paths. More particularly, the length of optical path b is changed by changing the location of the focusing lens, and the lengths of optical paths a and (a+b) are changed by adjusting the scanning optical system while changing the location of the focusing lens, whereby the original to be copied and the photosensitive medium is brought into a new conjugate relation. Such adjustment of the scanning optical system as well as the location of the focusing lens effected during the change of copy magnification is intended to minimize the amount of displacement of the focusing lens required to change the length of optical path a. For example, a variable magnification copying apparatus usually permits selection of the full-size copy or the reduced-size copy and when copy magnification is changed from the full-size to the reduced-size copy, the length of optical path a is increased. When such increment .DELTA.a in the length of optical path is realized only by the displacement of the lens, the amount of displacement thereof unavoidably becomes great and this practically raises an inconvenience in terms of precision. For example, if magnification is to be changed from the full-size copy to 0.7 times copy, the length of optical path a must be increased by 3f/7 while the length of optical path b only need be decreased by 3f/10.
There are known some copying apparatuses which are provided with first and second reflector means movable at a velocity ratio of 1:1/2 and permit change of copy magnification. For example, U.S. Pat. No. 3,614,222, U.S. Pat. No. 3,884,574 and U.S. Pat. No. 3,914,044 disclose copying apparatuses in which one end of a wire passed over a movable pulley for moving second reflector means at one-half the velocity of first reflector means is displaced to change the location of the second reflector means to a location corresponding to a selected copy magnification. In such apparatuses, it is indispensable to keep the end portion of the wire always under tension by a spring or like resilient means in order to prevent the wire from being slackened by the displacement of that end of the wire during the change of copy magnification. Therefore, such apparatuses have suffered from less freedom of design and have offered an inconvenience in that the tension imparted from the spring or the like to the wire end tends to vary with each change of copy magnification to make the scanning of the original unstable. Such resilient means for preventing the slackening of the wire during the change of copy magnification is unnecessary in the copying apparatus disclosed in Japanese Patent Publication No. 34731/1976. In this apparatus, however, both the forward and the backward movement of the first and second reflector means are accomplished limitedly by the use of a single wire and this reduces the degree of freedom of apparatus construction. Also, such apparatus employs a pulley for magnification change over which the wire is passed, the wire having substantially the opposite ends thereof substantially anchored to the apparatus body, and which is for displacing the second reflector means to a position corresponding to a selected copy magnification by a predetermined angle of rotation of the pulley, namely, by simultaneous substantial displacement of the substantially opposite ends of the wire, and thus, the apparatus requires a special mechanism for stopping the rotation of such pulley against the rotative drive imparted thereto from the wire when the first and second reflector means are moved forwardly to scan the original or are moved backwardly.
Further, in the known apparatuses mentioned above, the second reflector means is displaced in parallel by copy magnification change with the first reflector means remaining stationary in place. Any error in the location of such second reflector means, by which the light flux from the first reflector means is reflected in a direction substantially opposite to the direction of incidence from the first reflector means, would lead to an approximately two-fold error in the length of the optical path. In those known apparatuses, therefore, the tolerance of the displacement of the second reflector means by a copy magnification change operation is so small that the displacing mechanism must be of extremely high precision.